The present invention relates to a method and an apparatus for controlling the drive system for mobile equipment such as a mobile construction and/or mining machine, in particular mining truck, said drive system comprising at least two electric traction motors for driving separate wheels or separate crawler chains of said mobile construction and/or mining machine and a control unit for controlling speed, power output and/or torque of said electric traction motors, said control unit comprising an operator's drive commander for choosing a desired machine speed and/or desired power output.
Mobile construction and/or mining machines such as mining trucks or crawler chain vehicles often have an electric drive system including separate electric traction motors for individually driving some or all of the wheels wherein, when equipped with a crawler chain, said wheels may include driving wheels for driving the crawler chains. Usually a left traction motor is associated with a left wheel whereas a right traction motor is associated with a right wheel of the machine, wherein each traction motor can be associated with a single wheel or a pair of wheels on the left side or the right side. For some applications it has also been suggested to provide for each individual wheel a separate electric traction motor.
Advantageously, a control unit includes independent switch gear such as an IGBT or a GTO to provide variable speed and torque control independently to each motor. Electric energy can be supplied to the electric traction motors from an internal combustion engine such as a diesel engine driving an electric generator and limiting the total power output of the electric traction motors.
To drive the construction or mining machine at a desired speed, the machine's operator enters a respective command through a drive commander such as a pedal or possibly joystick to choose a desired drive torque or a machine speed or a desired power output such as “full speed”. In response to such command, which is commonly a torque command, the control unit controls or regulates the power output of the electric traction motors to drive the machine at the desired speed or at the possible speed at maximum capacity, for example, when taking a sloped road.
Beyond a certain speed, the drive capability of the electric traction motors is defined by a line of constant power up to a point where other drive related limitations such as the ratio of voltage to frequency V/Hz derate the curve below constant power. Traction motors operate at speeds that fall within the said constant power range speeds during the majority of operating time when the vehicle is moving.
FIG. 5 shows a typical speed/torque curve for an AC electric traction motor where it can be seen that in the aforementioned constant power range, the torque provided by the electric traction motor decreases with increasing speeds when the power supplied to the electric traction motor is kept constant. As can be seen from FIG. 5, the said constant power range is beyond the point of speed at which the motor provides its maximum torque output and defines a range of speeds where the torque decrease becomes smaller with increasing speed, that means the slope of the torque curve becomes less steep with increasing speed.
When 100% drive output is requested by the operator's drive command, typically, the instantaneous motor speed is captured and a torque command based of the curve is given independently of the other left or right drive. Normally, the left and right drives are speed synchronized via the ground so the speeds are the same and hence torques.
However, several factors can affect traction motor speed. The most relevant cases are (1) speed differential as a result of cornering and (2) individual motor speed fluctuations as a result of uneven ground, suspension and tyre dynamics.
In both cases, the resulting deviations of the traction motor speeds of the left and right drives cause torque differences when applying common drive control strategies.
For example, as shown in FIG. 6, when a speed difference is encountered during cornering, the left and right traction motors provide different torque when, according to a commonly applied control strategy, the control unit provides equal power to each traction motor, for example, when the drive is commanded to “full speed”. When the operator's drive command is kept constant, for example to 100% or to 75%, the possible points of operation of the electric traction motors are described by the curve shown in FIG. 6. In other words, when the drive command is kept constant, the point of operation of the electric traction motors may move on said curve shown in FIG. 6 upon variation of external load or resistance. More particularly, when speed decreases, the point of operation is shifted to the left and consequently, the output torque is increased since, as mentioned before, the traction motors are operated in the constant power range where the torque/speed curve shows the characteristic decrease of FIGS. 5 and 6. On the other hand, when the speed increases, the output torque provided by the traction motor decreases when constant power is applied to the motor.
Therefore, as can be seen from FIG. 6, during cornering the left and right traction motors provide different torques due to the deviation in motor speeds when equal power is provided to each traction motor. More particularly, the inside wheel rotates at a reduced speed during cornering, whereas the outside wheel rotates at an increased speed during cornering. Consequently, the inside traction motor provides a higher torque than the outside traction motor, cf. FIG. 6.
Such torque difference is of course unfavourable for the vehicle. It can result in                counter-steering torque, that means the torque difference opposes the turning direction,        uneven frame loading leading to an increased frame fatigue.        
Similarly, the aforementioned individual motor speed fluctuations resulting from uneven ground, suspension and tyre dynamics may cause deviations in motor speeds. Such traction motor speed changes due to truck dynamics are usually a phenomenon that affects an individual traction motor. During a dynamic event such as a bump in a road, a traction motor speed will experience sudden speed changes up to 50% depending on the severity of the event. In many cases, the events correspond to the resonant tyre or suspension frequency. Such changes in motor speed can cause torque fluctuations due to the characteristic torque speed behaviour shown in FIGS. 5 and 6 when equal power is supplied to the separate traction motors. Most often this can occur during traction or dynamic breaking events.
Such torque differences due to speed differences of the electric traction motors may cause stresses and oscillations in the frame and suspension elements, thereby reducing strength and in worst case creating microcracks in the structures.